1. Field of the Invention
The present invention relates to non-toxic ferrite dust for friction elements employed in brake systems of machinery such as vehicles, farming machines, heavy machines and trains, and more particularly to a raw material for producing such ferrite dust and a method of reducing the same.
2. Description of the Prior Art
Conventional friction elements employed in brake systems of various machinery such as vehicles, farming machines, heavy machines and trains have been made of acicular asbestos consisting predominately of SiO.sub.2. Since this material did severe harm to the human body, it has been highlighted as a severe problem of causing environmental pollution.
In advanced nations, reduced ferrite dust has already been used as the substitution for acicular asbestos, in order to not only improve the performance of friction elements, but also solve the problem of environmental pollution.
Friction elements should have superior friction characteristic and heat resistance, for a performance of products of brake systems.. Also, they should have a long use life and cause no wear damage at friction surfaces and no environmental pollution.
Accordingly, desirable conditions of Ferrite dust satisfying the above-mentioned requirements are a lower apparent specific gravity and a higher porosity. In particular, when the shape of grains is angular and the distribution of pores is irregular, there is an improvement in charge rate, upon a compact molding. In this case, the specific surface areas of grains are also increased, thereby enabling an increase in frictional coefficient and no occurrence of wear damage. Where ferrite dust is used alone for producing a friction element, however, a superior performance for the friction element can not be obtained. In this regard, ferrite dust has been conventionally added with nonmetallic oxides such as SiO.sub.2, CaO, Al.sub.2 O.sub.3, MgO, ZnO, P.sub.2 O.sub.5, MnO and etc., in proper amounts, so as to improve frictional characteristic, heat resistance and use life and avoid wear damage at friction surfaces.
One of known methods relating to reduced ferrite dust for friction elements is to reduce coarse mill scale with high FeO content, together with solid coke, pulverize the reduced mixture, and then subject it to a secondary reduction treatment, an annealing and a decarburization, so as to produce reduced ferrite dust.
However, such a method in which mill scale is used as a raw material for ferrite dust has disadvantages of a low productivity, in that it requires a repetition of crushing and pulverizing and a long treatment time for the reduction, the decarburization and the annealing. In particular, the mill scale is expensive, because it is used as a dephosphorization agent for molten iron.
Another method has been also proposed, which uses iron ore of high purity containing at least 95% magnetite (Fe.sub.3 O.sub.4) with a total Fe content of at least 70%. This method can reduce the treatment time, as compared with the method of reducing mill scale, in that the iron ore is reduced.
Ferrite dust by primarily reducing the iron ore together with solid coke. However, it encounters disadvantages in terms of supplying of raw materials and economy, because of difficulty in stably supplying high purity iron ore and instability in iron ore cost.
Referring to above description which has been made in conjunction with conventional method of producing reduced ferrite dust, it can be found that a raw material for producing friction elements is the criterion of evaluating workability, productivity, economy and environmental pollution caused by dust and that characteristic of raw material determines a reduction treatment method to be utilized.